It is known to form metallic protective layers on surfaces that are susceptible to carburization, for example on steel surfaces that are used in ultra-low sulfur reforming processes (see WO92/15653) and in other hydrocarbon conversion environments, such as hydrodealkylation (see WO94/15898). These patent applications teach the need for a separate cure step using pure hydrogen to form the metallic protective layer.
Unfortunately, unless there is a hydrogen plant nearby, obtaining pure hydrogen free of hydrocarbons is often difficult, and can be very costly. Moreover, when pure hydrogen is used, it is generally used in a once-thru manner. This is because hydrogen recycle is typically not possible, since most recycle gas compressors cannot handle low molecular weight gases, such as hydrocarbon-free hydrogen. To overcome this recycle problem, the pure hydrogen can be diluted with an inert gas (such as nitrogen). Then compression and recycle become doable. However, nitrogen is also difficult to obtain and costly. In summary, the need for once-thru hydrogen or adding an inert gas significantly adds to the cost of the cure step.
Yet the art for preparing and curing metallic protection layers teaches using a hydrogen stream that is free of hydrocarbons. For example, Heyse et al. in WO 92/15653 teach:
"The metallic coatings and, in particular, the paints are preferably treated under reducing conditions with hydrogen. Curing is preferably done in the absence of hydrocarbons." (page 25, line 23-5, emphasis added.) PA1 1. Heating the reactor system to the cure temperature (typically between 600-1800.degree. F.) in a hydrogen atmosphere; PA1 2. Holding at the cure temperature under hydrogen for up to 3 days; PA1 3. Cooling the reactor system; and only then PA1 4. Beginning standard process start-up procedures.
An almost identical teaching can be found in Heyse et al. WO 94/15898 on page 23, lines 5-7. Both these patent applications are incorporated herein by reference, especially with regard to useful coating materials and process conditions for curing.
With the known curing process, the start-up procedure, for example, after painting or applying a metal-containing coating to a steel substrate, includes:
The new process of this invention eliminates the first three of these steps; it uses "normal", "standard", or only slightly modified start-up procedures--that is, start-up in the presence of feed--to form the metallic protective layer in-situ. It does not require a separate and time-consuming cure step using pure or hydrocarbon-free hydrogen. Thus, the new process reduces start-up times by up to three days and increases on-stream time.
The new process of this invention is especially useful for touch-up situations. For example, it may be used to form a metallic protective layer on a section of a furnace tube that needs replacement. The tube is brought off-line, then cut out and replaced with a new steel section. This section is coated or painted with a metal-containing coating, and then welded in place. As the tube comes on-stream and heats in the presence of hydrocarbon feed, the protective layer is formed in-situ.